Preheat the oven to 350°F and grease a pie pan. Unroll the pie crust and press down into the pie pan. Press against the sides and the bottom. Top the crust with something heavy (beans, rice, ceramic weights) and bake for 12 - 15 minutes until the edges of the crust just begin to turn golden.
To make the lemon filling, to a medium saucepan add 1 cup sugar, flour, cornstarch, salt, and whisk. Add water, lemon juice, and zest and bring the mixture to a boil. Add butter.
To a small bowl add egg yolks and gradually add ½ cup of hot sugar mixture.
Add egg yolk mixture to the remaining sugar mixture, bring to a boil and continue cooking until thick. Transfer the filling to a baked pastry shell.
To make the meringue, to a large glass or metal bowl add egg whites and whip until foamy.
Slowly add sugar, and continue whipping until stiff peaks are formed.
Spread meringue over pie, sealing the edges at the crust. Bake in preheated oven for 10 minutes.